The present invention relates to global positioning systems (GPS) and in particular, to any satellite navigation system. It is also generally directed to direct sequence spread spectrum (DSSS) code-division multiple access (CDMA) communication systems.
Satellite navigation systems are systems which use a plurality of satellite vehicles (SVs) to provide accurate timing signals and navigation data which can be utilized by a navigation receiver to determine the range between the receiver and a satellite. By determining the range for at least four SVs, three-dimensional position location via intersection of the associated ranging spheres can be used to determine the three-dimensional location of the receiver; that is, the position of the receiver with respect to the surface of the earth as well as its height relative to the surface of the earth (the height may represent the elevation of the receiver on the earth or the height of the receiver above the earth in situations where the receiver is in some type of aircraft or the like; the fourth SV is necessary for local clock correction).
The concept underlying range determination is time-of-arrival (TOA) ranging, that is determining a position between the source of a signal and a receiver receiving the signal by measuring the propagation time of the signal from the transmitter to the receiver. In order to accomplish this TOA determination, it is necessary that SVs accurately and repetitively generate timing signals based upon a system clock which is effectively synchronized to the receiver system clock. It is thereby possible to determine the length of time it takes the signal to traverse the distance between the transmitter (satellite) and the receiver, and by knowledge of the propagation speed of the propagating signal (typically the propagation speed of the speed of light, c), it is possible to determine the distance between the satellite and the receiver.
Through similar range information for at least four SVs, the intersection of the three ranging spheres denotes two points (locations), one of which is the correct position of the receiver. The two candidate locations are mirror images of one another with respect to the plane of the three SVs and for a user on the earth""s surface, it is therefore apparent that the lower location will be the receiver""s true position. For users above the earth""s surface, such unambiguous determination is not always possible without ancillary information. General information concerning the satellite navigation system deployed by the United States (which is commonly referred to as GPS), as well as the satellite navigation system deployed by Russia""s Ministry of Defence (known as the Global Navigation Satellite System-GLONASS-) is presented in Understanding GPS Principles and Applications, E. D. Kaplan, Editor, Artech House Publishers, Copyright 1996.
With respect to the discussions herein, the acronym GPS will generally refer to any type of satellite navigation system.
In order to generate such timing signals and navigation data which can be used by the receiver to determine the position of the SV at the time of generating a timing signal, the satellite system employs a characteristic frequency methodology. In particular, GPS satellite signals include frequency assignment, modulation format and the generation of pseudo-random noise (PRN) codes. Each GPS satellite transmits two carrier frequencies, L1, the primary frequency, and L2, the secondary frequency. The carrier frequencies are modulated by spread spectrum codes with a unique PRN sequence associated with each SV and at least one carrier frequency is further modulated by the navigation data. All GPS SVs transmit at the same two carrier frequencies but their signals are effectively non-interfering with each other due to the unique PRN codes that are essentially non-correlating to each other. Thus the PRN code sequences for each of the SVs are nearly uncorrelated with respect to each other and thus the SV signals can be separated and detected by a technique known as code-division multiple access (CDMA). Detailed discussion of this modulation technique is presented in Chapter 4 of Understanding GPS Principles and Applications. The specific PRN CDMA codes used for the GPS SVs are sometimes referred to as Gold codes.
As discussed earlier, the GPS receiver""s primary task is the measurement of range and range rate (that is the change in range) between itself and each of a plurality of visible SVs. To perform this task the satellite receiver must also demodulate the received navigation data. The navigation data consists of a 50 bits per second data stream which is modulated onto the GPS PRN modulated signal. Navigation data contains the SV clock information as well as the orbital elements for the SV, the latter elements used to compute the position of the SV at the time of generation of the received PRN code. Details concerning fundamentals of satellite orbits can be found in Section 2.3 of the above-identified text, Understanding GPS Principles and Applications. 
The GPS signal modulation format is known as direct sequence spread-spectrum. An overview of GPS signal processing is presented in Braasch, M. S., xe2x80x9cGPS Receiver Architectures and Measurements,xe2x80x9d Proc. of the IEEE, Vol. 87, 1. Jan. 19, 1999, pp 48-64. As there discussed, in a spread-spectrum system the data are modulated onto the carrier such that the transmitted signal generally has a much larger bandwidth than the information rate of the data. The spread spectrum system typically uses a deterministic signal known to the receiver (the PRN code) which is used by the transmitter to modulate the information signal and spread the spectrum of the transmitted signal. Finally, the spread-spectrum system uses a receiver which cross correlates the received signal with a copy of the deterministic signal in the process of demodulating the data and by so doing, the transmitted data is recovered.
The type of spread-spectrum used by the GPS satellite system is binary phase shift keying direct sequence spread-spectrum (BPSK DSSS). The term xe2x80x9cdirect sequencexe2x80x9d means that the spreading of the spectrum is accomplished by phase modulation of the carrier. Binary phase shift keying is the simplest form of phase modulation where the carrier is instantly phase shifted by 180 degrees at the time of a bit change.
Thus during normal navigation operation, a GPS receiver must adjust a local deterministic signal (replica code), which for GPS receivers is the PRN CDMA code, in order to determine a match of this PRN CDMA code with the PRN code received and thereby identify the satellite as well as the timing signal which is embodied in the PRN code received. In addition, the receiver must employ a Doppler Compensation Circuit for each satellite being received.
The operation of the continuously adjusting the local replica PRN CDMA code generator is known as code phase tracking, while the Doppler Compensation Circuit adjustment for each SV is known as carrier phase tracking. The tracking sub-systems are often referred to as tracking loops in order to emphasize the fact that tracking involves a closed-loop control system. As noted earlier, what makes the GPS satellite system different from typical CDMA communication systems is the fact that the DSSS spreading code is not only a modulation method or multiple access method but is also a technique used to accurately measure the propagation delays from each SV to the receiver. It thus represents a timing signal which is used to determine the range between each satellite and the receiver.
Algorithms are used by GPS receivers that can estimate the delay (time of arrival) of each SV code with accuracy which is better than the number of bits in the spreading code, which for civilian GPS use is 1,023 bits per second. This PRN spreading code is known as course acquisition (C/A) (a higher frequency xe2x80x9cPxe2x80x9d code is also transmitted by the SVsxe2x80x94this P code has a frequency ten times that of the C/A code, although the P code is encrypted and is generally not available for civilian use). In order to emphasize that these 1,023 bits do not represent data bits, these bits are normally referred to as xe2x80x9cchipsxe2x80x9d in GPS parlance. Thus the specific PRN code sequence for each SV is repetitively transmitted by the specific SV every millisecond.
Since the course acquisition (C/A) code has a 1 millisecond period which repeats constantly, the code length effectively limits the precision of the range determination. To achieve sub-chip delay measurement accuracy, it is necessary to take into account, multipath propagation effects in order to mitigate these effects. Multipath propagation effects occur when the signal from the SV to the receiver is received in not only the direct unobstructed path, but along other paths such as reflection off of buildings, surrounding structures, aircraft and the like. The most harmful multipath components are components with delay differences of 0 to 1.0 chip corresponding respectively to 0 nanoseconds and 1 microsecond delays. At the propagation speed of light (c), these chip delays correspond to 0 meter and 300 meter path differences respectively.
Thus the resistance of the code phase detector of the satellite receiver to multipath contaminated signals is a very important characteristic that dominates range determination and thus positioning accuracy.
The present invention is directed to minimizing the effects of multipath disturbances of GPS signals so as to maintain precise timing with respect to the received PRN CDMA code. Multipath components of the SV signal are components which do not take a direct path to the receiver but rather arrive later than a direct line path. Such multipath components can result from reflections caused by aircraft, buildings, terrain and the like and effectively degrade the received GPS signal. Since each multipath component arrives at the receiver after the straight line signal (that is non-reflected signal), the multipath components do not affect the starting point of the leading edge of the correlation function. The correlation function is the correlation between the received PRN CDMA code and the replica PRN code generated at the receiver which is used to determine when the PRN CDMA code has arrived at the receiver. It is observed that a comparison of the amplitude of the correlation function at the beginning of the leading edge of the correlating region to the amplitude of the correlation function in a non-correlating region should yield a value of zero. However, it is also observed that this initial point (at xe2x88x921 bit or as commonly called xe2x88x921 chip) begins to climb up the leading edge of the correlating region if multipath components of the received signal are present. This initial (xe2x88x921 chip) point movement is due to multipath skewing of the correlation triangle. This initial point movement can be measured and used to compensate for multipath effects.
The present invention uses this information concerning movement of the initial point of the correlation function due to receipt of multipath components of the CDMA code to compensate for such components and thereby correct the location of the start of the PRN CDMA code. This start point is the point of maximum correlation and is called the prompt (P) correlator. Thus the multipath component effects on the exact timing of the receipt of the PRN CDMA code is minimized, thereby giving greater accuracy to the range determination between the SV and the receiver.